The contribution to the US energy supply from unconventional gas sources is growing dramatically. Shale gas production in the US has increased from 0.3 TCF in 1996 to 1.1 TCF in 2006, accounting for 6% of the nation's supply.
Water is used extensively in shale gas production. A typical well consumes 4-5 million gallons of water during the drilling and hydrofracturing processes. Typically this water is trucked in from remote locations. In addition, gas after the hydrofracturing process, much of this water is returned the surface as a brine solution termed “frac flowback water”, which is followed by “produced water”. Both flowback and produced water will be referred to as “frac” water. Because the frac water has very high salinity (50,000-200,000 ppm TDS), it cannot be disposed of in surface waters. Frac water is frequently disposed of in salt-water disposal wells, which are deep injection wells in salt formations. A significant problem with many of the shale gas plays, including the Marcellus Shale, is that there are few available deep well injection sites wells and environmental regulations prohibit discharge to rivers and other surface waters. In other shale gas plays, such as the Barnett Shale in Texas, water availability is limited and the use of large quantities of water for gas production generates much resistance from the public. Therefore, technology that enables cost-effective water reuse in shale gas production is essential for sustained development of this resource.
In current water management models for shale gas operations using hydraulic fracturing, fresh water is injected to form cracks in the formation and deliver proppant to maintain permeability. In addition to proppant, a number of chemicals are added to the water to reduce friction, minimize corrosion and scale, prevent bacterial growth, and reduce drilling mud damage. About 20-50% of the water used to hydrofracture a well is returned as flowback, usually within a 2-3 weeks of injection. This water contains high amounts of dissolved solids (TDS ˜50,000-200,000 ppm for the Marcellus shales), in addition to many of the injected additives. It is stored in suitable containment tanks before being transported to appropriate disposal facilities. Historically, off-site disposal has been the more favorable wastewater treatment option. This is because the relatively small scale of water production and short time frame associated with drilling make capital investment in stationary local treatment options impractical. In recent years, there have been efforts to develop economically viable on-site treatment and recycling using mobile water treatment facilities.
The process of drilling and preparing a natural gas well in a gas shale formation typically requires 4-5 million gallons of fresh water per well. The cost to treat and dispose of this water is a significant expense to the gas producers. Brine disposal usually entails shipping the wastewater to an old well for reinjection. Treatment of the water on or adjacent to the well site would eliminate the cost of shipping millions of gallons of wastewater to a treatment facility and/or disposal site, and obviate the need to truck in additional millions of gallons of fresh water.
In the presence of sulfate ion, a solution of RaCl2 and BaCl2 coprecipitates as RaSO4 and BaSO4 even when the concentrations of Ra++ and SO4= are far below the solubility of RaSO4 in water. Radium and barium coprecipitate to form a solid solution of RaSO4—BaSO4. Under ideal solid solution conditions, the calculated solubility of RaSO4 in equilibrium with a solid solution of RaSO4—BaSO4 is 8.2×10−12 M, or 1.8×10−6 ppm Ra. This solubility of Ra is equivalent to 1800 pCi/L of 226Ra. The presence of NaCl enhances the coprecipitation of Ra with BaSO4.
The sulfate treatment method generates a BaSO4/RaSO4 sludge. This sludge may be combined with lime treatment sludge. The sludge stream must be dried, and then hauled to a landfill. The cost for sludge disposal as nonhazardous waste in a RCRA-D landfill is typically about $50/ton. However, to qualify for disposal as nonhazardous waste, the sludge must have an activity below a value of 5 to 50 pCi/gm (varies by state). The maximum activity for nonhazardous waste disposal in Pennsylvania is 25 pCi/gm. Sludge that exceeds this value needs to be disposed of as low-level radioactive waste (LLRW), which is discussed below. If the radium activity exceeds about 400 pCi/L, the sludge will need to be either blended with sufficient non-radioactive solid waste to meet the RCRA-D specification or treated as low-level radioactive waste (LLRW). The cost of LLRW disposal is too high for a sulfate precipitation process to be used for frac water from many, if not most, sites.
Radium selective complexing (RSC) resins are a strong acid gelular cation exchange resins that have been completely exchanged with barium followed by sulfuric acid treatment to make finely dispersed, bound BaSO4 microcrystallites. These bound BaSO4 crystallites ion exchange with radium as shown below.RaCl2(aq)+BaSO4(resin)→RaSO4(resin)+BaCl2(aq) 
RSC resin is often utilized to remove radium from brine that is used to regenerate softening ion exchange resins in municipal water systems. For example, in a demonstration study (Mangelson, K. A. and Lauch, R. P., Removing and Disposing of Radium from Well Water, Journal of the American Water Works Association, 82 (6), 72-76 (1990)), hardness (Mg++, Ca++, Sr++, Ba++) and radium were removed from drinking water using a conventional gelular sulfonic acid ion exchange resin (sodium form). The softening ion exchange resin was regenerated with NaCl brine (average 40,600 ppm TDS) to remove hardness and radium. The regeneration brine leaving the ion exchange resin contained an average of 1180 pCi/liter radium. Prior to disposal, this regeneration brine was treated with RSC resin. The RSC resin removed an average of 99.2% of the radium from the softener regeneration brine. After one year of operation, the resin loading was 3,000 pCi/cc resin.
However, the results of some studies have indicated that RSC resins are not suitable for use with water containing the very high levels of total dissolved solids that are typically found in frac water. See, for example, Snoeyink, et al. (1987 Environmental Research Brief, Removal of Barium and Radium from Groundwater, February 1987), and references cited therein.
Therefore, there remains a need for cost effective methods to treat frac waters before reusing or recycling.